The level of certain analytes in blood and other body fluids can predict disease or risk thereof. For example, the amounts of cholesterol and triglycerides in blood are a significant indicator of risk of coronary heart disease (“CHD”). Excess triglycerides in plasma is called “hypertriglyceridemia,” and is linked to the occurrence of atherosclerosis and CHD in some people. Elevated triglycerides may be a consequence of other disease, such as untreated diabetes mellitus. Like cholesterol, increases in triglyceride levels can be detected by plasma measurements.
A number of enzymatic methods have been developed for determining triglycerides in plasma or serum. One method involves the following reaction pathway.
   
Dry-phase test strips are known which rely upon the above reaction pathway and which include the reagents necessary for reaction loaded into their reaction or test membranes. In these test strips, blood cells are separated from plasma, and the resulting plasma is then passed into one or more test membranes. Color is produced in the test membrane and is read by reflectance photometry and correlated to concentration of triglycerides. In a dry-phase test system, all of the reagents to carry out the color producing reaction are typically loaded into a single test membrane. Thus, the pH of the aqueous solution used to load the reagents in the test membrane must be compatible with all reagents.
In the above pathway, the reagents 4-aminoantipyrine (4AAP) and lipoprotein lipase (LPL) are critical. Commercially available LPL is typically derived from pseudomonas. LPL starts the reaction above by hydrolyzing the triglycerides into their component fatty acid and glycerol molecules as shown above. LPL typically requires a pH range of 6–9 for its activity. On the other hand, the Trinder reagent 4AAP, which is responsible for color formation as shown in the last step of the above pathway, is not stable in alkaline environments. Generally speaking, the lower the pH, the more stable is 4AAP. Thus, there is a delicate balance of maintaining the pH of the impregnating solution high enough to maintain the activity of LPL on the one hand, and maintaining the pH low enough to maintain the stability of the 4AAP on the other hand. Stability of triglyceride strips is a problem, and the best commercially available triglycerides test strips known to applicants require storage at 4° C. to maintain their stability for even a couple of months. In these prior art test strips, the test membrane is impregnated with a solution whose pH is maintained at pH 6.2–6.7 . Further, an aqueous Klucel Citrate foundation is mixed separately and added to the impregnating solution, and the foundation's pH is also maintained at pH 6.2–6.7.
Prior art FIGS. 1 and 2 illustrate that, prior art triglycerides test strips made by the assignee of the present invention are not stable for a significant length of time above room temperature. To generate the graphs shown in FIGS. 1 and 2, a control set of prior art test strips was stored at 4° C., whereas other prior art test strips were stored at 25° C., 35° C. and 45° C., respectively. Triglycerides concentration in whole blood samples was measured at various time intervals using the refrigerated strips as a control and the other strips stored at elevated temperatures. The ratio of the concentration of triglycerides measured from strips stored at 4° C. versus the strips stored at 25° C., 35° C. and 45° C. are plotted in FIGS. 1 and 2. FIG. 2 includes samples known to have a higher concentration of triglycerides than the samples plotted in FIG. 1. As can be appreciated, serious degradation in test result occurs within 4 weeks of storage for the strips stored at 35° C. and 45° C. For the strips stored at 25° C., the results are stable for a longer duration, but begin to degrade after 20 weeks of storage as shown.
It can be appreciated that test strips may spend several weeks in storage at the manufacturer's facility before they are transported. During transportation, if the strips are shipped without being refrigerated, the temperatures to which they are exposed can be quite high, up to 45° C. As can be appreciated, prior art triglycerides test strips cannot withstand such high temperatures during transportation. Thus, it would be desirable to enhance the stability of triglyceride test strips at room temperatures and elevated temperatures such that they can be shipped with other non-refrigerated items and stored in a room temperature environment.